More than 250,000 people have already died in the brutal civil war which began in March 2011.

But U.S. Secretary of State John Kerry about Russia's military support for the al-Assad regime could raise the risk of confrontation with coalition forces fighting Islamic State there.

In an interview with Italian newspaper La Stampa, Kerry said he had told Russian Foreign Minister Sergei Lavrov the United States was worried by Moscow's military backing for Assad in Syria's civil war, now in its fifth year.

"These actions could provoke a further escalation of the conflict and lead to the loss of more innocent lives, increasing the flow of refugees and risking a confrontation with the anti-ISIS coalition operating in Syria," Mr Kerry said.

He did not spell out what he meant by the coalition.

But the United States and allies have been launching air strikes against positions held by the Islamist militant group in Syria, and in neighbouring Iraq.

Israel has also raised concerns that Russian involvement on Assad's side in the conflict in Syria could accidentally lead to confrontations between Russian and Israeli forces there.

The involvment of Russia has also been criticised by Gulf states who are also opposed to Assad's regime and its alley Iran.